Respiratory syncytial virus (RSV), subgroups A and B, is the leading cause of pediatric respiratory tract diseases worldwide. RSV is also responsible for serious respiratory tract infection in the elderly and immunocompromised individuals. No vaccine is currently available to prevent severe diseases caused by RSV infection. The recent published report from the Institute of Medicine lists RSV vaccine as a level II vaccine priority to develop. Aviron proposes to develop a live attenuated vaccine for the prevention of diseases associated with RSV infection. In the SBIR phase I application, we proposed to attenuate recombinant RSV using recently developed "reverse genetics" technology. Several promising candidates have been generated and many more mutants are currently being characterized in vitro. In phase II of the SBIR program, we propose to screen potential attenuated RSV vaccine candidates in suitable animal models and to initiate human clinical trials. Two to four attenuated RSV mutants will be prepared for clinical studies. The ultimate goal is to select a RSV subgroup A and B vaccine strain, based on human clinical trials, for final commercial development. PROPOSED COMMERCIAL APPLICATION: NOT AVAILABLE